Technological advances in various fields allied to video recording have caused many researchers to focus on high resolution and high definition television systems, referred to here as HDTV systems. HDTV systems offer the advantages of extremely high picture quality, significantly larger TV displays, and high quality sound. The movement toward HDTV systems involves a transition from known analog TV systems to digital systems, essentially as has occurred in the audio field in moving from vinyl phonograph records to digital compact disc technology.
Analog TV systems, which include most of the TVs in use today, create the picture or TV image using a varying voltage that controls the position and characteristics of an electron beam. The electron beam is systematically swept across the internal side of the TV screen from left to right and down, much as a person reads English language print on a page. As the electron beam strikes the surface of the TV screen, light is produced which is seen collectively by the viewer as the TV image.
Analog TV technologies are inherently limited in several respects. Probably the most important limitation is the inability to accurately detect and correct errors in the analog signal. The analog TV signal is broadcasted, for example, by a local TV station, as a radio wave with varying voltage. This broadcasted signal can be distorted by environmental or other disturbances prior to reaching the TV receiver. The signal also can be distorted in the TV circuitry. Because there is no reliable reference to detect and correct errors, these errors can produce imperfections or distortions in the resulting TV image.
Another limitation of analog TV systems is the practical limitation on the data rate, i.e., the effective rate at which the TV signal carries data or picture and sound information. Analog TV systems produce the TV image by rapidly projecting a series of still images or image frames, in essentially the same way a movie film is projected with a series of frames to create a moving picture. The analog TV signal is divided into segments or frames corresponding to the projected frames of the TV image. The full frame of the analog signal is necessary to construct the corresponding full TV image frame. It is difficult or impossible to transmit the analog frames in a more compact form, for example, by eliminating redundant information from frame to frame.
Digital TV systems overcome these limitations by using a digital signal that includes numerical data for each picture element or pixel of the TV image frame. The digital signal is segmented into a series of digital frames, with each frame including a series of numbers, ones and zeros, corresponding to the pieces of information in the frame. The word "frame" as used in the remainder of this document refers to a digital frame unless otherwise indicated.
Digital technology offers a number of advantages, some of the most important of which are error detection and correction features. The information in the digital signal also can be processed to reduce or eliminate redundancies from frame to frame of a TV image, and the digital information can be coded (using a single symbol or small group of symbols to represent a larger number or set of numbers) to increase the efficiency of the data transfer and correspondingly to increase the data transfer rate. A processor in the digital TV set can be used to decode the data and use it to project the desired TV image. Techniques for increasing the efficiency of the data and correspondingly decreasing the redundancy are known as data compression techniques.
As a means to achieve desired high data rates, various data compression techniques have been proposed. Compression techniques involving "interframe coding" and "variable length coding" appear to be particularly attractive for digital video applications. Interframe coding involves reducing or eliminating frame-to-frame image redundancies by using motion vectors (numerical data representing motion in the TV image) and residual data (data representing the difference between the TV image constructed using motion vectors and the actual image data obtained from the camera during encoding) instead of fully reproducing the data of each TV image frame. Variable length coding reduces or eliminates frame-to-frame redundancies in the data stream by using variable length digital codes instead of the actual data. With interframe and variable length coding, each digital frame includes two types or pieces of data--intraframe (IA) data and interframe (IR) data. IA data within a given frame is data that is unique to that frame. IR data is data that relates to or links two or more frames, e.g., from one frame to the next. Motion vectors and residual data are examples of IR data. Using these types of techniques in digital TV applications, compression factors of about 30-40 have been reported. With these compression factors, signals, e.g., at 750 megabits per second (Mb/s) can be broadcasted as variable length coded and error protected data streams at data rates of 19-25 Mb/s.
Applying digital technology to VCR equipment, and particularly to VCR equipment that is practical for consumer markets, presents a number of challenges. Most importantly, the cost of the systems must be relatively low for market acceptability.
In addition, TV decoders must be able to receive data streams from one or more sources. Because video compression systems with interframe coding schemes usually encode current frame data in reference to the data in preceding frames, it is necessary that a decoder be able to start decoding interframe data at any point in reference to data of preceding frames. To start decoding these interframe data at any point of a continuous data stream without an existing reference frame, the information required to construct an initial reference frame must be transmitted along with the interframe data. Because construction of an initial frame cannot be done instantly, the display can be very visibly interrupted if concealment techniques are not applied.